A sigma-delta converter typically comprises a sigma-delta modulator and a digital filter. The analogue signal to be digitized is applied as input to the modulator, and is sampled thereby at a relatively high frequency (in relation to the maximum frequency of the input signal), called oversampling frequency. The modulator produces, at the oversampling frequency, binary samples representative of the analogue input signal. The output bit stream from the sigma-delta modulator is processed by the digital filter which extracts from it an N-bit digital value (N being the quantization resolution of the sigma-delta converter), representative of the input signal. The number of binary samples (that is to say the number of oversampling periods) necessary to produce an N-bit digital output value is designated by the acronym OSR, from the acronym “Over Sampling Ratio”.
The sigma-delta modulator typically consists of a loop comprising at least an analogue integration circuit, a 1-bit analogue-digital converter, a 1-bit digital-analogue converter, and a subtractor. The analogue input signal is applied to the input of the integration circuit, which samples it at the oversampling frequency and supplies, at this same frequency, analogue samples representative of the difference between the input signal and an analogue feedback signal. The analogue output samples from the integration circuit are digitized by the 1-bit analogue-digital converter (typically a comparator). The binary samples thus obtained from the output signal of the modulator. These binary samples are elsewhere converted into analogue samples by the 1-bit digital-analogue converter, the analogue signal thus obtained forming the feedback signal of the modulator. The analogue integration circuit can comprise a single analogue integrator, or several cascaded analogue integrators. It can also comprise one or more subtractors, one or more summers, and/or one or more weighting coefficients. The number p of analogue integrators generally defines the order of the sigma-delta modulator. The higher the order p of the modulator, the more the number OSR of samples necessary to obtain a digital output value on N-bits can be reduced (given identical quantization noise levels). On the other hand, the sigma-delta modulators are all the more complex to produce when their order is high (stabilization is difficult).
The digital filter comprises, depending on the structure of the modulator, one or more digital integrators (generally at least as many as there are analogue integrators in the modulator), for example counters, and performs a filtering function intended to extract the useful information from the bit stream produced by the sigma-delta modulator. More particularly, the sigma-delta modulator formats the useful signal via its signal transfer function STF, and the quantization noise via its noise transfer function NTF. The STF is the transfer function linking the analogue input signal to be digitized to the output signal of the modulator, and the NTF is the transfer function linking the quantization noise introduced by the 1-bit analogue-digital converter of the modulator on the output signal of the modulator. The NTF makes it possible to push back the quantization noise outside of the band of interest (in which the signal is located). The digital filter is designed so as to extract the signal in the frequency bands in which the attenuation of the quantization noise by the NTF is high (that is to say where the signal is located). The signal transfer function STF is generally equal to 1, and the noise transfer function NTF is expressed, for example, for a modulator of order p, by NTF(z)=(1−z−1)p.
There is a need to at least partly improve certain aspects of the existing sigma-delta converters.